FIG. 1 is a diagram of a circuit set forth in U.S. patent application Ser. No. 10/788,180, filed on Feb. 25, 2004 (the subject matter of which is incorporated herein by reference). In the circuit, a transmitter LED within an IrDA (Infrared Data Association) module is used both to make IrDA transmissions and to make remote control (RC) transmissions. Although IrDA uses an infrared signal of approximately 875 nm and RC remote control devices use an infrared signal of approximately 960 nm, the IrDA transmitter LED actually transmits over a range of wavelengths. By increasing the transmission power of the IrDA LED, the amount of energy transmitted at 960 nm can be made sufficiently large that the IrDA transmitter LED can be used in an RC transmitter mode to make RC transmissions to an RC receiver.
FIG. 2 is a diagram that shows the wavelengths transmitted by the 875 nm IrDA LED. Note that the curve of the IrDA transmitter LED overlaps the response curve of the RC receiver.
FIG. 3 is a diagram that shows how the current through the IrDA 875 nm transmitter diode is increased in the RC transmitter mode until the IrDA module outputs enough energy at 900 nm for RC receiver operation.
The circuit of FIG. 1 works by taking advantage of the fact that RC signals are transmitted with longer bursts of IR energy (approximately 10 microseconds), whereas the IrDA signals are transmitted with shorter bursts of IR energy (approximately 1.6 microseconds). During a short IrDA pulse, an inductor L in the current limiting circuit of FIG. 1 is not conducting a significant amount of current. Current flow through the current limiting circuit is limited by the current limiting resistor R1 in the left leg of the current limiting circuit. The value of R1 is chosen to limit the current flow into the LED to a relatively low current level suitable for the IrDA transmission.
During a relatively longer RC pulse, the inductor L of the current limiting circuit of FIG. 1 conducts increasingly more current. A magnetic field builds up in the inductor until the core of the inductor saturates. When the inductor L saturates, current flow through the current limiting circuit is limited by the equivalent resistance of R1 in parallel with the current limiting resistor R2 in the right leg of the current limiting circuit. This peak amount of current is the current flow into the LED. The value of R2 is chosen to limit the current flow into the LED to the higher current required for the RC transmission.
The circuit of FIG. 1 may be used in personal digital assistants (PDAs). In a PDA, battery life is often a very important design consideration. Being able to extend battery life is very valuable. A technique is sought whereby power consumption can be reduced. In both the IrDA mode and the RC mode, current is being limited by a resistance, and the current flow across the resistance dissipates energy.
FIG. 4 is a simplified diagram of the IrDA LED circuit. The diagram illustrates power losses in the current flow across the current limiting resistance and across the on-resistance of the field effect transistor (FET) of the IrDA module. It would be desirable to reduce these losses.